Retort containers are hermetically sealed after filling with a food product, and are then heated to a temperature of at least about 220° F. for a period of time to ensure that all microorganisms in the container have been killed. Cycle times for heating or cooking the container contents typically exceed 10 minutes. As the temperature rises, the pressure inside the container also increases. An overpressure control system is often used to reduce the pressure differential between the inside and outside of the container. As such, the pressure differential typically does not exceed 5 pounds per square inch (psi). After heating, retort containers typically do not require refrigeration prior to opening and consuming the contents. Retort containers can be stored for extended periods in their sealed condition.
For many years, metal cans were the predominant type of retort containers. Plastic retort containers have been developed. In plastic retort containers, the container lid must be hermetically sealed to the container with sufficient strength to withstand the elevated temperature and pressure conditions during the retort operation. However, the seal strength must be limited so that the consumer may readily remove the lid and seal. Other requirements for all-plastic retort containers and lids include high-barrier performance against water vapor and oxygen, and fast sealing speed.
An all-plastic retort container having a sealing membrane that is adhesively sealed to the container (as opposed to being heat-sealed to the container) is disclosed in Nomula US 2005/0255266 A1.
A high-barrier plastic closure for a retort container is disclosed in Nomula U.S. Pat. No. 7,364,779 and Williams et al, US2005/0145630 A1.
A composite closure for a retort container is disclosed in German et al. U.S. Pat. No. 6,276,543. The closure includes a metal disk for covering the container opening. The disk is retained by an outer band that is threaded onto the rim of the container.